Blender does not utilize full amount of VRAM
Unless you have multiple graphics cards you are unlikely to be able to max out the memory use.  Some of the memory will be used by other processes and windows.  I've found it is always worth closing all other windows and any other unnecessary processes when I'm close to the limit.  They all use RAM which could affect your render but they all will likely have staked a claim on some VRAM too.  I don't know whether minimising the Blender program windows makes any difference but I do it anyway.  If you are on Windows I've also found that switching themes can help.  Current Windows versions and their UI effects are not a particularly efficient users of VRAM so dropping back to the Classic theme will probably help.  I've also read that changing font size to normal and switching off clear type and anti-aliasing can help although I've no evidence to support that.